Global navigation satellite systems (GNSS) are broadly defined to include GPS (U.S.), Galileo (proposed), GLONASS (Russia), Beidou (China), IRNSS (India, proposed), QZSS (Japan, proposed) and other current and future positioning technologies using signals from satellites, with or without augmentation from terrestrial sources. Information from GNSS is being increasingly used for computing a user's positional information (e.g. a location, a speed, a direction of travel, etc.). Typically, a GNSS receiver tracks signals from multiple satellites in one or more satellite systems and uses information included in the signals to compute the positional information.
In order to compute the positional information, the GNSS receiver needs to know a position of each of the satellites it is tracking and a satellite time. A satellite signal transmitted by a satellite may be composed of ephemeris data, e.g., a set of values that provides the positions of an astronomical object at a given time, which may enable the GNSS receiver to know the position of the satellites it is tracking. The satellite signal also periodically transmits the current satellite time in the form of the time counter which increments by a fixed amount with every successive transmission. A periodicity of successive transmissions is 12 seconds for GPS L2-C signals and 30 seconds for Galileo signals. In the context of a GPS receiver, the time counter may be a time of the week (TOW) value, which is a time stamp that may be present in a time interval of the satellite signal that provides a seventeen-bit counter time value reset each week. In both GPS L2-C and Galileo signals, the time counter is convolutionally encoded prior to transmission. At low signal to noise ratios it may be difficult to reliably decode the value of the time counter using only one instance of the time counter.
Furthermore, at lower satellite signal powers the process of demodulation of the data becomes increasingly unreliable. This primarily affects the GNSS receiver operating in the Autonomous Cold Start mode at lower signal powers. The GNSS receiver must decode the convolutionally encoded time counter before it can compute the positional information. The process of decoding the time counter for obtaining the timestamp (current satellite time) is commonly referred to as “decoding time”. Since the GNSS receiver cannot compute the positional information until the time counter is decoded, it is highly desirable that the GNSS receiver decodes the time as fast as possible even in constrained environments such as indoors where signals received from the GNSS satellites may be weak.